<?php
function buildRandomString($type = 1,$length = 4){

	if($type==1){
		$chars = join("",range(0,9));
	}elseif($type==2){
		$chars = join("",array_merge(range("a","z"),rang("A","Z")));
	}elseif($type==3){
		$chars = join("",array_merge(range("a","z"),range("A","Z"),range(0,9)));
	}
	if($length > strlen($chars)){
		exit("字符串长度不够");
	}

	$chars = str_shuffle($chars);
	return substr($chars,0,$length);

}
/**
 * 生成唯一字符串
 * @return string
 */
function getUniName(){
	return md5(uniqid(microtime(true),true));
}
/**
 * 得到文件的扩展名
 * @param string $filename
 * @return string
 */
function getExt($filename){
	return strtolower(end(explode(".",$filename)));
}
